It is often necessary or desirable to measure various parameters of blood, such as temperature and blood constituents, such as blood gases, pH, other electrolytes and glucose. This can be accomplished in real time using fluorescent sensors. For example, this can be accomplished in an extracorporeal blood loop as shown in Cooper U.S. Pat. No. 4,640,820 and in vivo as disclosed in Lubbers et al Reissue Pat. No. 31,879. For in vivo sensing a probe or catheter carrying an appropriate sensor is inserted into a blood vessel of the patient. Because blood vessels are quite small, sensors designed to be inserted in such vessels must be very small. This size constraint may have a detrimental effect on the accuracy of the determination made by the sensor.
One of the most important gases that needs to be sensed is oxygen. One problem with in vivo oxygen sensing is that the readings obtained for the concentrations of oxygen tend to vary over an unacceptably wide range when compared with the results obtained using conventional laboratory techniques for measuring the concentration of oxygen. It has been found that this deviation is in many cases unacceptably large so that the reliability of the in vivo measuring system is called into question. Clearly, it would be advantageous to provide a system having many of the benefits of an in vivo measuring system while reducing or eliminating one or more of the deficiencies apparent in prior art in vivo systems.
Kater U.S. Pat. No. 4,535,786 discloses a method and apparatus for measuring parameters of body fluids, e.g., blood, using sensing electrodes which are calibrated periodically. Kater discloses a reversible pump, e.g., a two circuit positive displacement pump, to alternately pump calibration fluid, e.g., a modified and buffered ringer solution, from a source into contact with a reference electrode and, with the pump reversed, pump blood from the patient into contact with the sensing electrodes for taking measurements. In between measurements, the reversible pump is reversed again and operated at a reduced rate to pump a small amount of calibration fluid into the patient to keep the blood vessel open. Stopping the pump would stop the flow of calibration fluid. Kater discloses that all other infusions of liquid into the patient which come in contact with the electrodes are discontinued while blood is being drawn into contact with the electrodes. It would be advantageous to use sensors which did not require repeated calibration. Also, the use of a single reversible pump to pass calibration fluid into contact with a reference electrode at one flowrate, and into the patient at a different flowrate and to reverse and draw blood from the patient at a third flowrate, places an over-reliance on a single piece of equipment and/or requires relatively sophisticated equipment. A simpler system would be less costly and more reliable.